8.4*28+8.4*72
=8.4×（28＋72）
=8.4×100
=840
1.25/5+0.25*20
=0.25＋5
=5.25
4.75+8.4/2.4*20
=4.75+21/6×20
=4.75+70
=74.75
7.6*0.25*4
=7.6×（0.25×4）
=7.6×1
=7.6